Interfacial and Aqueous‐Phase Oxidation of Salicylic Acid by ·OH Radicals

Salicylic acid (SA, C 7 H 6 O 3 ) is an aromatic compound widely used in medicine and industry. Its low solubility and partial dissociation in water affect its environmental persistence and transport via atmospheric aerosols, leading to its increasing detection in seawater and sea spray aerosols. Recent European Union regulations have heightened interest in its chemical transformations in natural systems. This study investigated the degradation of SA initiated by ·OH radicals either (1) spontaneously generated at the air–water interface of SA aerosols at 25°C, or (2) in the bulk aqueous phase, where they were produced by UV photolysis ( λ > 300 nm) of H 2 O 2 or NO 3 − at 25°C. Aerosols were generated by nebulization, stabilized through alkaline deprotonation (pH 8.2), oxidized in a flow tube, collected onto filters and analyzed via ultra‐high‐performance liquid chromatography coupled with high‐resolution mass spectrometry (UHPLC‐HRMS), and at the bulk‐phase products at pH 4.5 were identified also by UHPLC‐HRMS. At the air–water interface, spontaneous dark oxidation yielded maleic acid, catechol, and dihydroxybenzoic acids within a 90‐s timescale. The bulk‐phase chemistry under simulated solar irradiation produced a consistent suite of products; notably, H 2 O 2 /UV and NO 3 − /UV systems also generated trihydroxylated derivatives, triggering a temporary toxic activation window flagged by REACH Annex III regulations. These results demonstrate that SA undergoes radical‐mediated oxidation in both phases through highly spatially decoupled mechanisms, highlighting its complex role in atmospheric multiphase chemistry.